1. Field of Invention
This invention relates to printing and particularly to print hammer devices for use in high speed printing apparatus.
2. Description of Prior Art
Print hammer devices have heretofore comprised a pivoted single print hammer element in which the armature or yoke of an electro-magnetic actuator and the hammer portion are structurally combined. In some cases the armature and hammer are made as an integral single piece so as to reduce cost and simplify manufacture. Examples of such print hammer devices are described in the following U.S. Pat. Nos. 3,177,803--Antonucci; 3,200,739--Antonucci; 3,349,696--Potter; 3,513,773--Ponzano; 3,705,370--Chai, et al.; 3,711,804--Kroft, et al.; 3,714,892--Perry; 3,747,521--Hamilton, et al.; and the following publications; IBM Technical Disclosure Bulletin, pages 3529-31, Vol. 16, #11, dated April, 1974 and pages 780-81 of Vol. 17, #3, dated August, 1974.
All of the above references are concerned with short contact or dwell time and/or elimination of double impact, both of which degrade the quality of printing produced in high speed printer apparatus. A common solution proposed by the prior art for essentially rigid hammer elements is to use, an elastic stop member in the vicinity of the hammer head or stem portion of the hammer or a return spring or both as described in the Antonucci and Potter patents, a permanent restore magnet as described in the patents of Kroft et al. and Chai et al., or to lump mass at critical positions relative to the impact point as disclosed in the referenced publications.
Another solution proposed by the patents of Hamilton et al., Ponzano and Perry is to provide an elastic hammer element in which the hammer portion continues to move after the armature is abruptly stopped by contact with the pole pieces of the electro-magnet.
While the above described structures provide means to obtain good print quality at relatively high speeds, certain difficulties arise when the print speeds are to be further increased. The rigid hammer structures are essentially too heavy to have a short enough contact time and require high energy. Energy which can be used for printing is lost to return springs or penetration bars. Likewise, in the elastically deformable hammer substantial energy is lost to the elastic deformation prior to impact. Furthermore, impact of the armature on the residual or pole piece in the operating air gap of the electro-magnet over a period of time causes degradation of the visco-elastic and non-magnetic properties of the material. This eventually produces erratic operation for the hammers thereby affecting print quality. It is also difficult to find materials in which the non-magnetic properties and visco-elasticity of the residual are the most desirable to fulfill both functions for rebounding the armature, etc.